Not applicable.
Not applicable.
The present invention relates to a process and furnace for melting glass forming ingredients. In the typical glass melting furnace, or glass tank as it is commonly referred to, the raw glass making materials, termed batch, are charged into the melting zone of the furnace. Glass tanks are operated continuously and therefore there is an existing bath of molten glass, termed melt, in the melting zone onto which the raw material is placed. The molten glass and un-melted batch are collectively referred to as the charge. The raw batch may be charged into the tank by any of the well-known mechanical charging devices. In practice, the batch materials float on the surface of the molten bath forming a semi-submerged layer containing un-melted solids termed a batch blanket. The blanket sometimes breaks up to form separate batch piles or batch islands (also called rafts or logs). For the purposes of this disclosure, the section of the furnace containing significant unmelted batch solids floating on the surface of a molten glass bath is defined as the melting zone.
The glass tank usually consists of the melting zone and the fining zone. For the purpose of this disclosure, the fining zone is defined as that section of the furnace not containing significant un-melted batch solids floating on the surface of a molten glass bath. Foam or scum may be present on the surface of the molten glass bath in the fining zone or it may be clear, termed xe2x80x9cmirror surfacexe2x80x9d glass. In the fining zone, glass is homogenized and defects, such as bubbles or xe2x80x9cseedsxe2x80x9d are driven out. Glass is continuously withdrawn from the fining zone. The melting zone and the fining zone of a glass tank may be present in a single chamber or the glass tank may consist of two or more connected and distinct chambers.
Glass has historically been melted in air-fuel furnaces where burners direct flames across molten glass and the exhaust gas from the flames is removed through heat recovery devices to improve the overall furnace efficiency, thereby reducing fuel consumption. Recuperators and regenerators are common heat recovery devices used in the glass industry. A recuperator is typically a metallic shell-and-tube-style heat exchanger that indirectly heats the combustion air with the heat removed from exhaust gases. In the case of regenerators, the exhaust gas passes through the regenerators transferring its heat to the checker packing or other heat storage media within the regenerator. The checker packing is generally constructed from refractory material. The regenerator may be a common chamber per each side of the furnace, a number of separate and distinct chambers attached to the furnace or may be incorporated into the burner supply ducting. The heated packing is used to preheat combustion air which is combined with fuel used to produce the flames during the firing cycle of the heating operation. These heat recovery devices are costly and sometimes limit the furnace life due to design limitations, failure caused by thermal shock to the refractory, corrosion, or plugging. Occasionally glass is melted in a unit melter which is a furnace without a heat recovery device to preheat combustion air.
In the case of regenerators, the thermal storage medium, i.e. the checkers, become plugged by condensed volatiles and particulates from the glass melting process, resulting in insufficient flow of combustion air to the ports. Consequently, glass manufacturers routinely clean out the checker packs to maintain air flow. The plugging problem is noticeably worse for the ports connected to the melting zone of the furnace. The buildup in the regenerator packing which is contacted by gases from the melting zone of the furnace is often viscous and difficult to remove. Controlling buildup of material on the checker packing of a regenerator is the subject of U.S. Pat. No, 5,840,093. The buildup in the down tank checkers which is contacted by gases from the fining zone of the furnace is drier and more powdery resulting in easier removal of the buildup. Because of the less aggressive attack, down tank checkers have been used for more than one furnace campaign.
Near the end of a furnace campaign, it is sometimes the case that the checkers become too badly degraded, at times even collapsing, and sufficient air flow is not possible even after a clean out. The problem usually manifests itself in the regenerator section receiving gases from the melting zone of the furnace. Oxygen enhanced combustion technologies have been used in these xe2x80x9ccrippledxe2x80x9d air-fuel furnaces to extend the furnace life. While the oxygen enhanced combustion technologies do not prevent the checker plugging problem, they do provide a method to continue furnace operation, albeit sometimes with a higher operating cost.
Industrial oxygen has been used to enhance combustion in the glass industry for several decades. Oxygen enhanced combustion can be accomplished by (i) supplemental oxy-fuel burners, (ii) premixed oxygen enrichment of the combustion air, or (iii) lancing of oxygen to the port or burner. Supplemental oxy-fuel is the practice of installing one or more oxy-fuel burners into an air-fuel furnace. Premixed oxygen enrichment is the practice of introducing oxygen into the combustion air usually to a level of up to 30% total contained oxygen (i.e., 9% oxygen enrichment). The amount of oxygen enrichment is limited by materials compatibility issues in highly oxidizing environments. Lancing is the practice of strategically injecting oxygen through a lance into the combustion zone. These oxygen enhancing techniques are applied to furnaces with burners having standard air-fuel designs. The basic air-fuel furnace concept has not been significantly modified to apply the above mentioned oxygen enrichment technologies.
Supplemental oxy-fuel combustion has been applied to air-fuel glass furnaces and has shown benefits. One form of supplemental oxy-fuel combustion is commonly referred to as oxy-fuel boosting. Oxy-fuel boosting is a technology where oxy-fuel burners are added to an air-fuel furnace. Two locations for the oxy-fuel burners have been proposed: near the hot spot position and in the zero port position. Typically the oxy-fuel burners fire constantly, even during the reversal cycle of a regenerative furnace.
The rationale for putting the oxy-fuel burners in the hot spot position is to reinforce the hot spot with additional heat to positively influence the convective flow patterns in the glass melt and, as described in several patents, to affect the position of the batch-line. The overall glass flow pattern is strongly influenced by buoyancy driven flow and the temperature profile in the furnace is important for the buoyancy driven flow. Ultimately the glass quality is affected. This is why glass-makers control and monitor the temperature profile in a furnace.
Similar to the hot spot oxy-fuel boost, U.S. Pat. No. 3,592,623 discloses a process and furnace where at least part of the furnace heating is provided by an oxy-fuel flame from a position downstream of the hot spot. The combustion products of the flame impinge on the un-melted glass making materials (i.e. the batch) causing the un-melted materials to remain near the feed end of the tank until melted. An objective is to control the position of the un-melted batch material (batch-line) in the glass tank. The remaining heating is provided by air-fuel combustion as shown in the figures of ""623.
U.S. Pat. No. 4,473,388 discloses an oxy-fuel boost process where the oxy-fuel flames cover substantially the whole width of the furnace and are directed at the batch-line.
U.S. Pat. No. 5,139,558 discloses a process where at least part of the furnace heating is provided by at least one flame from at least one oxy-fuel burner located in the roof of the furnace, the position of the burner being such that the tip of its flame is directed approximately at the batch-line. An objective of both of the ""623 and ""558 processes is to increase the melting rate of the solid glass forming materials and control the batch-line position.
Oxy-fuel firing over the down tank molten glass in a regenerative or recuperative air-fuel furnace is the subject of U.S. Pat. No. 5,116,399. The object of this disclosure is to use an oxy-fuel flame with velocity greater than 100 m/sec to sweep un-melted glass forming ingredients floating on the surface of the melt in the vicinity of the glass outlet to prevent any un-melted glass forming ingredients from entering the glass outlet. Use of supplemental oxy-fuel burners combined with the oxy-fuel burner for sweeping unmelted glass forming ingredients within the air-fuel furnace configuration is also disclosed.
Supplemental oxy-fuel boost of an air-fuel regenerative furnace is disclosed in U.S. Pat. No. 5,147,438 where the oxy-fuel auxiliary burner is bent, angled, or inclined to direct its flame toward the batch-line or in the vicinity of the batch-line.
As an alternative to the hot spot position, oxy-fuel boost can be placed at the charge end of a furnace. In a side-fired furnace, this is commonly referred to as the zero port position. It is the space between the charge end-wall and the first air-fuel port. The rationale for this location is higher heat transfer rates from the hot oxy-fuel flames to the cold batch. Zero port oxy-fuel boosting is a common method used by industry and is described in Hope and Schemberg (1997). This reference teaches that as a result of more intense radiant heat transfer to the cold batch from the oxy-fuel boost flame, earlier batch fritting and glazing occurs than is possible with just air-fuel melting. The percentage of oxy-fuel firing for the zero port boosting technology has been approximately up to 15% of the total firing rate and is often limited by the maximum allowable temperature of the superstructure refractory. Using zero port oxy-fuel boosting, production increases on the order of 5 to 10% have been achieved with simultaneous glass quality improvement.
U.S. Pat. No. 4,531,960 teaches zero port oxy-fuel boosting where the supplemental (auxiliary) oxy-fuel flames are surrounded with a current of auxiliary gas where the auxiliary gas is preferably air and directing the auxiliary gas towards the batch clods (un-melted batch piles or islands). One of the objectives of the auxiliary gas is to eliminate the use of water cooling of the oxy-fuel burner which was a common feature of oxy-fuel burners at the time of the patent filing. Those skilled in the art of NOx technologies will readily appreciate that practice of this teaching would result in an increased NOx formation since the nitrogen from the air would mix in the high temperature oxy-fuel flame.
Practice of these supplemental oxy-fuel technologies without high NOx formation rates, requires special methodologies.
An extension of the supplemental oxy-fuel technologies would be to combine the zero port and hot spot oxy-fuel boosting to capture the benefits of improved melting in the melting zone and batch-line control. This process, however, is likely to particularly cause concern over an increase in the propensity for NOx formation because there will be more of the hotter oxy-fuel flames available to form NOx with the migrating nitrogen from the air-fuel combustion section. Thus, a process utilizing both air-fuel and oxy-fuel combustion in the same furnace without taking into consideration the problem of NOx formation would be an incomplete solution.
To minimize formation of NOx associated with increasing supplemental oxy-fuel in air-fuel furnaces, there has been a general trend in the glass industry to convert from air-fuel firing to full oxy-fuel firing. In this manner, the NOx forming nitrogen is eliminated as part of the feed to the burners. Because of this NOx issue and other issues, the move to full oxy-fuel is the obvious choice if increasing use of oxy-fuel is desired especially at levels suitable for an on-site generated supply of oxygen. In contrast to oxygen enhanced combustion technologies, significant modifications are made to a furnace to apply full oxy-fuel combustion in furnaces. In full oxy-fuel furnaces, air for combustion is replaced by industrial oxygen with purity typically between 90 and 100%. Heat recovery devices used in air-fuel furnaces such as regenerators and recuperators are generally not used after the furnace has been converted to oxy-fuel. Different burners and flow systems are used and the general layout of the burners and exhausts is almost always different than previous air-fuel furnace designs.
Full oxy-fuel firing in a glass furnace is a demonstrated and proven technology.
Eleazer and Hoke in chapter 7 titled Glass, of the publication Oxygen-Enhanced Combustion, Charles E. Baukal, Jr., Editor, 1998 pp. 215-236 report 110 full oxy-fuel conversions in North America. Implementation of full oxy-fuel combustion in glass furnaces is the topic of U.S. Pat. Nos. 5,417,732 and 5,655,464. Some of the benefits reported for oxy-fuel operation are; fuel savings due to improved furnace efficiency, production increase resulting from improved heat transfer, reduced electricity costs by substituting combustion energy for electric boost energy, extended furnace life by overcoming combustion air throughput limitations caused by plugged checkers or a failing recuperator, extended furnace life by substituting combustion energy for electric boost energy, thereby reducing refractory wear caused by electric boost, reduced pollutant emissions such as NOx, particulates, and carbon dioxide, improved glass quality resulting from improved furnace temperature profile, lower volatilization, better batch-line control, and decreased capital cost by reducing or eliminating post-treatment systems and/or heat recovery systems.
However, the use of full oxy-fuel is not without problems or concerns. The atmosphere generated by oxy-fuel firing over a glass melt has been found to be more aggressive to superstructure refractory than an air-fuel atmosphere. Several articles in the 57th Conference on Glass Problems in 1996 discussed the increased corrosion of superstructure refractory resulting from oxy-fuel firing. Consequently, new construction techniques and new materials of construction which are often more expensive, have been proposed for oxy-fuel fired furnaces. In addition to the obvious concern for furnace integrity, refractory corrosion can be detrimental to glass quality if the liquefied superstructure refractory gets into the glass.
Operators of oxy-fuel glass furnaces have reported an increase in foam on the glass surface as compared to air-fuel operation. Foam is believed to have a negative impact on heat transfer and on glass quality. Heat transfer is affected because foam has poor conductive properties. U.S. Pat. No. 3,350,185 addresses the problem of foam formation and the elimination thereof.
Increased heat transfer using oxy-fuel is the subject of PCT International Patent Application WO 99/31021 which describes-a roof-mounted oxy-fuel burner process and furnace to produce refined glass without the use of regenerators or recuperators. This application describes high level usage of oxy-fuel combustion with impingement on the batch surface in the melting zone of the furnace for increased heat transfer. This application teaches the use of at least one roof-mounted oxy-fuel burner in the fining zone for combustion proximate top surface melted raw glass-forming material to reduce the layer of foam from the melted glass surface to aid in refinement of the melted glass. At least one roof-mounted oxy-fuel burner in the fining zone is said to have been found to improve the quality of the glass moving forward into the forming area by removing surface defects such as incompletely reacted raw glass-forming material or insufficiently mixed surface materials by substantially raising the surface glass temperature, promoting melting and mixing. Furthermore, at least one downstream oxy-fuel burner provides a barrier to the forward flow of material, promotes natural convection currents within the molten glass causing hotter glass to flow backwards under the raw glass-forming material thereby preventing a forward surge of the molten glass, increasing the melting effect and increasing the glass temperatures in the fining zone. As this is a full oxy-fuel furnace technology, this patent application states that NOx emissions are reduced compared to an all air-fuel furnace technology.
The operation of a full oxy-fuel furnace depends on the constant availability of oxygen. Many of the larger oxy-fuel glass furnaces are supplied by oxygen generated on site using well-known cryogenic distillation or vacuum swing adsorption techniques. It is customary and, to date, the only method for backing up the supply of on-site generated oxygen by maintaining an inventory of liquid oxygen at the same site. Thus, when the on-site generation facility is taken off-line either due to a process problem or for routine maintenance, the inventory of liquid oxygen is utilized to supply the oxygen for the oxy-fuel combustion. This method of backing up the on-site generated oxygen requires large insulated tanks for storage of the oxygen in liquid form and vaporizers to enable the liquid oxygen to be converted into gaseous oxygen for use in the oxy-fuel process. It is conventional to utilize trucks to haul liquid oxygen to the site from a larger air separation facility. Utilizing liquid oxygen back-up with an on-site generated oxygen system permits the user to continue using an oxy-fuel process without interruption. An alternative technology, where air-fuel combustion, with and without oxygen enrichment, is used to back up an oxy-fuel furnace is described in co-pending U.S. Pat. No. Application Serial No. 09/420,215 filed Oct. 14, 1999.
Since the furnace efficiency changes over the life of the furnace using full oxy-fuel combustion and the glass production rate may vary over the furnace campaign, the associated oxygen generator is typically sized for the maximum planned usage rate. This results in an underutilized oxygen generator for a large percentage of the furnace campaign.
McMahon et al. (hereafter xe2x80x9cMcMahonxe2x80x9d) in an article entitled xe2x80x9cCan Partial Conversion to Oxy-fuel Combustion be a Solution to furnace Problemsxe2x80x9d (Glass Industry, December 1994) teaches a partial conversion of an air-fuel furnace to an oxy-fuel furnace prior to and as part of a complete re-build to an oxy-fuel furnace. A key in McMahon is that the ports in the previous heat recovery system were left open in the converted section of the furnace""s partially converted state, notwithstanding that oxy-fuel combustion does not require heat recovery. (Oxy-fuel combustion, burning hotter than air fuel combustion, does not require recovering heat from the combustion products exhausted from the furnace in order to preheat the combustion reactants and boost the combustion temperatures.)
Leaving the ports open in McMahon in the converted section was related to the control of NOx emissions. In particular, this allowed air to enter the converted section, thereby facilitating the aspiration principle whereby the entering air and furnace gases are integrated with the oxy-fuel burners such that the oxygen and fuel come into contact only at low concentrations, thereby yielding peak flame temperatures equal or lower than those of regenerative air/fuel flames, thereby reducing NOx. The air entering the converted section brings into question, however, whether oxy-fuel combustion (defined in the Application as combustion where the oxidant stream is between 50 and 100% oxygen and preferably between 90 and 100% oxygen) occurred in the converted section or, as is more likely according to Applicant""s calculations, only xe2x80x9coxygen-enrichedxe2x80x9d combustion occurred in the converted section.
Leaving the ports open in McMahon in the converted section also mitigated the migration of combustion products from the converted section to the unconverted section, thereby impeding the batch volatiles (which are commingled with the oxy-fuel combustion products from the converted section) from entering the unconverted section and being exhausted from the unconverted section and subsequently plugging and/or corroding the heat recovery system needed for the unconverted section. Leaving the ports open in the converted section, however, also allows combustion products from the unconverted section to exhaust from the converted section, thereby allowing the nitrogen contained in the air used for the air-fuel combustion in the unconverted section to enter the converted section and mix with oxygen and subsequently form NOx.
None of the references discussed above teach a process or furnace with predominantly oxy-fuel combustion heating in the melting zone and predominantly air-fuel combustion heating in the fining zone.
The present invention is a process and furnace for melting glass wherein the majority of the combustion energy over the melting zone of the furnace is provided by oxy-fuel combustion while a majority of the combustion energy over the fining zone of the furnace is provided by air-fuel combustion.